Application of the Hydra Assay for Rapid Detection of Developmental Hazards

A broad-taxa approach as an important concept in ecotoxicological studies and pollution monitoring

Aquatic invertebrates play a pivotal role in (eco)toxicological assessments because they offer ethical, cost-effective and repeatable testing options. Additionally, their significance in the food chain and their ability to represent diverse aquatic ecosystems make them valuable subjects for (eco)toxicological studies. To ensure consistency and comparability across studies, international (eco)toxicology guidelines have been used to establish standardised methods and protocols for data collection, analysis and interpretation. However, the current standardised protocols primarily focus on a limited number of aquatic invertebrate species, mainly from Arthropoda, Mollusca and Annelida. These protocols are suitable for basic toxicity screening, effectively assessing the immediate and severe effects of toxic substances on organisms. For more comprehensive and ecologically relevant assessments, particularly those addressing long-term effects and ecosystem-wide impacts, we recommended the use of a broader diversity of species, since the present choice of taxa exacerbates the limited scope of basic ecotoxicological studies. This review provides a comprehensive overview of (eco)toxicological studies, focusing on major aquatic invertebrate taxa and how they are used to assess the impact of chemicals in diverse aquatic environments. The present work supports the use of a broad-taxa approach in basic environmental assessments, as it better represents the natural populations inhabiting various ecosystems. Advances in omics and other biochemical and computational techniques make the broad-taxa approach more feasible, enabling mechanistic studies on non-model organisms. By combining these approaches with in vitro techniques together with the broad-taxa approach, researchers can gain insights into less-explored impacts of pollution, such as changes in population diversity, the development of tolerance and transgenerational inheritance of pollution responses, the impact on organism phenotypic plasticity, biological invasion outcomes, social behaviour changes, metabolome changes, regeneration phenomena, disease susceptibility and tissue pathologies. This review also emphasises the need for harmonised data-reporting standards and minimum annotation checklists to ensure that research results are findable, accessible, interoperable and reusable (FAIR), maximising the use and reusability of data. The ultimate goal is to encourage integrated and holistic problem-focused collaboration between diverse scientific disciplines, international standardisation organisations and decision-making bodies, with a focus on transdisciplinary knowledge co-production for the One-Health approach.

Alternative Tests for Teratogenicity

Summary

The major features of the tests surveyed are shown in Table I. In a tier system of tests for teratogenicity, the Chernoff test is at a different level than the other assays described here. It is not appropriate for screening large numbers of chemicals, but may be useful for studies of smaller groups of agents, for example to confirm data from a prescreen. Although the test is certainly easier, cheaper and uses less than half the animals of a Segment II test, it is still much more expensive and time-consuming than most alternative tests.

Of the remaining alternatives, whole embryos or organs in culture encompass the widest range of mammalian developmental events and are invaluable in the study of teratogenic mechanisms. They are, however, also inappropriate for screening large numbers of chemicals. The methods are technically demanding, relatively expensive and use reasonably large numbers of pregnant mammals. To screen a group of, say, 20 chemicals involves a considerable investment of time and, in fact, no study of this size has been reported. In certain specific circumstances, they may be a useful adjunt to testing; for example, if treated human serum samples are freely available, if a drug has a unique action on rodent dams which confounds evaluation of the standard in vivo tests, or if human metabolism is important and can be mimicked in vitro.

Sub-mammalian and sub-vertebrate species offer considerable advantages; reduced cost, relative rapidity and no requirement for laboratory animals. FETAX provides some indication of teratogenicity in relation to embryotoxicity, while CHEST and the planarian and Drosophila assays measure only teratogenic potential, or more strictly speaking, embryotoxic potential, although it should be possible to derive some assessment of hazard with each of the latter systems. The Hydra system is cheap, quick and easy and is commercially available. It is the only assay specifically designed to estimate teratogenic hazard and may offer considerable advantages as an alternative screen.

The metabolic cooperation assay has not generated sufficient data to enable evaluation. The neural crest cell assay is not well developed as a routine screen, and objective endpoints which are not measures of general cytotoxicity must be devised. The viral morphogenesis and Drosophila embryo cell assays have both produced encouraging validation data. With further assessment, the viral system may be shown to be useful, but it is a relatively complex assay and its relevance to teratogenesis is obscure. The Drosophila system is easier, has been used with more chemicals and is developmentally relevant. However, it has not produced dose-response data to evaluate potency or hazard, and must be improved so that it can more clearly distinguish cytotoxicity. The measurement of endpoints in the neuroblastoma cell line assay requires further refinement, and contributions of growth inhibition or stimulation to effects on differentiation must be examined. In combination, tumour cell attachment and HEPM may prove valuable. Alone, HEPM appears to be an assay for cellular toxicity, not teratogenicity, and the attachment assay suffers from a high rate of false negatives because it measures only one cell phenomenon. Although micromass cultures use mammalian tissue, are not the cheapest assays and require some skill for full evaluation of the results obtained, they show considerable promise. Validation data are encouraging, the assay includes several developmental processes and the use of multiple endpoints permits specific developmental toxicities to be evaluated.

Ecotoxicity responses of the freshwater cnidarian Hydra attenuata to 11 rare earth elements

Lanthanides are the major family of rare earth elements (REEs) owing to the essential properties these metallic species provide in diverse fields of today's world economy. They are now being mined and produced as never before. This raises new environmental concerns in terms of their expected future discharges notably to aquatic systems. Interspecies studies of their ecotoxicity are sparse and effects on aquatic life are still poorly understood. Absence of such information for cnidarians, an ecologically relevant freshwater community, thus prompted the present research on REEs toxicity using Hydra attenuata as our animal model. Lethal and sublethal ecotoxicity data generated with the 11 REEs displayed LC50 values ranging from 0.21 to 0.77 mg L^-1and EC50 values ranging from 0.02 to 0.27 mg L^-1, thereby confirming the inherent sensitivity of Hydra to REE exposure at environmentally relevant concentrations. Additionally, two properties of REEs were shown to modulate Hydra (sub)lethal toxicity (LC50 and EC50) which decreases with increasing atomic number and with decreasing ionic radius. Compared to studies carried out with different taxonomic groups, Hydra toxicity responses to REEs proved to be among the most sensitive, along with those of other invertebrate species (i.e., Daphnia magna, Ceriodaphnia dubia, Hyalella azteca), suggesting that members of this community are likely more at risk to eventual REE discharges in aquatic environments. Demonstrated Hydra sensitivity to REE exposure strongly justifies their future use in toxicity testing battery approaches to evaluate liquid samples suspected of harbouring REEs.

Developmental Toxicity: Extrapolation Across Species

There is a pervasive lack of understanding about the definition of a developmental toxicant and the experimental design necessary to differentiate between maternal toxicants and those uniquely, i.e., selectively, hazardous to the conceptus. Thus, the acceptance of the adult-to-developmental (A/D) ratio and its ability to identify those compounds uniquely hazardous to the conceptus has been compromised, as has the extrapolation of hazard across species. The failure to establish a shared understanding is discussed, together with the role of an in vitro screen for determining the A/D ratio. The addition of an environmental exposure factor permitting the extrapolation of hazard assessment across species and the estimation of potential human developmental toxicity is detailed.

An Analysis of the Hydra Assay's Applicability and Reliability as a Developmental Toxicity Prescreen

A large and diverse group of chemicals was evaluated in the Hydra assay, and the outcomes were compared with those of standard developmental toxicity evaluations using data from pregnant mammals. Hydra correctly identified each of the substances previously found by in vivo tests to be uniquely hazardous to in utero development. Its overall accuracy was over 90%, and all of its errors were false positives. The fact that these positives were indeed false could have been established subsequently by routine testing in mammals. In no instance did the assay indicate that a chemical was not uniquely hazardous to the conceptus when higher level tests indicated that it was, i.e., 0 false negatives. The possible areas for continued refinement of the assay and expansion of its scope of use as well as perceived problems and limitations are discussed in detail.

Aquatic ecotoxicity assessment of a new natural formicide

BACKGROUND, AIM AND SCOPE

Agrochemicals could reach aquatic ecosystems and damage ecosystem functionality. Natural formicide could be an alternative to use in comparison with the more toxic formicides available on the market. Thus, the objective of this study was to assess the ecotoxicity of the new natural formicide Macex® with a battery of classical aquatic ecotoxicity tests.

MATERIAL AND METHODS

Bacteria (Aliivibrio fischeri), algae (Pseudokirchneriella subcapitata), hydra (Hydra attenuata), daphnids (Daphnia magna), and fish (Danio rerio) tests were performed in accordance with international standardized methodologies.

RESULTS

In the range of formicide concentrations tested (0.03 to 2.0 g L(-1)) EC(50) values varied from 0.49 to >2.0 g L(-1), with P. subcapitata being the most sensitive species and H. attenuata and D. rerio the most tolerant species to this product in aqueous solutions.

CONCLUSIONS

This new formicide preparation can be classed as a product of low toxicity compared to the aquatic ecotoxicity of the most common commercialized formicides.

Primary cell and micromass culture in assessing developmental toxicity

Under the European Commission's New Chemical Policy both currently used and new chemicals should be tested for their toxicities in several areas, one of which was reproductive/developmental toxicity. Thousands of chemicals will need testing which will require a large number of laboratory animals. In vitro systems (as pre-screens or as validated alternatives) appear to be useful tools to reduce the number of whole animals used or refine procedures and hence decrease the cost for the chemical industry. Validated in vitro systems exist for developmental toxicity/embryotoxicity testing. Indeed, three assays have recently been validated: the whole embryo culture (WEC), the rat limb bud micromass (MM), and the embryonic stem cell test (EST). In this article, the use of primary embryonic cell culture, and in particular micromass culture, including a relatively novel chick heart micromass (MM) culture system has been described and compared to the validated D3 mouse embryonic stem cell (ESC) test.

Activity profiles of developmental toxicity: design considerations and pilot implementation

The available literature was searched for quantitative test results from both in vitro and in vivo assays for developmental toxicity for five model compounds: cyclophosphamide, methotrexate, hydroxyurea, caffeine, and ethylenethiourea. These compounds were chosen on the basis of their extensive utilization in a variety of assay systems for developmental toxicity as evidenced by their representation in the ETIC database (each generally has 100-500 citations encompassing multiple test systems). Nine cellular-based assays, six assays using whole embryos in culture, as well as Segment II and abbreviated exposure tests for mammalian test species are included in the database. For each assay, the critical endpoints were identified, each of which was then provided a three-letter code, and the criteria for extraction of quantitative information were established. The extracted information was placed into a computerized reference file and subsequently plotted such that the qualitative (positive/negative) and quantitative (e.g., IC50, highest ineffective dose (HID), lowest effective dose (LED] results across all test systems could be displayed. The information contained in these profiles can be used to compare qualitative and quantitative results across multiple assay systems, to identify data gaps in the literature, to evaluate the concordance of the assays, to calculate relative potencies, and to examine structure-activity relationships.

In vitro differential developmental toxicity of vitamin A congeners

Several forms of vitamin A were tested in the in vitro hydra assay for their developmental toxicity hazard potential and site of action on progressive ontogenesis. Retinol, retinyl acetate, retinaldehyde, all trans retinoic acid, and 13 cis retinoic acid were tested fully, and each was established clearly as being able to perturb development of artificial hydra "embryos" at, or near, adult toxic treatment levels. All forms of vitamin A tested interfered with differentiation, but, although the alcohol, acetate, and aldehyde forms (group I) prevented the initial stages of differentiation from occurring, the acid forms (group II) allowed the initial stages of differentiation to occur but not the final differentiation of tentacle buds. Group I compounds produced the developmental toxicity endpoint after as little as 24 h of transient exposure on the first day of development, but had no permanent effect on development at their minimal affective developmental concentration (D-MAC) when exposure began after the first day of development. In contrast, transient 24-h exposure to group II forms did not interfere with development. At, or even above, a concentration greater than the D-MAC, more continuous exposure to them was required to interfere with differentiation. Consistent with tests of other chemicals, the concentrations needed to produce effects in hydra bore no relation to those needed to produce effects in mammals.

A tier system for developmental toxicity evaluations based on considerations of exposure and effect relationships

The backlog of untested chemicals and the rate at which new substances enter the marketplace exceed our capacity for developmental effects testing by standard in vivo methods. However, conservative use of two observations in a manner consistent with present day understanding of abnormal developmental biology can more accurately focus attention and resources on those agents in greatest need of complex testing for effects on in utero development. These two observations are 1) most chemicals are no more toxic to embryonic development than they are to adult homeostasis and 2) most human exposure to chemicals is de minimus, i.e., so small that it is inconsequential. Recently devised in vitro assays to quantitatively rank chemicals according to their developmental hazard index, when used in conjunction with more conventional in vivo methods and appropriate considerations of exposure, permit evaluation of a significantly larger number of chemicals than is currently achieved. The methods described apply a tier approach to establish testing priorities that markedly reduce the time, cost, and number of laboratory animals needed for evaluation of developmental toxicity.

Approaches to defining the relationship of maternal and developmental toxicity

Maternal and developmental toxicities reported in the literature were examined in an attempt to define more clearly their relationship. Relationships were difficult to ascertain because maternal toxicity end points are not clearly defined, or even assessed, in every study. However, maternal toxicity accompanied by developmental toxicity is the most common outcome of in vivo testing. Approaches to define these associations have included assessment of acute maternal toxicity and teratogenicity; evaluation of maternal toxicity and its association to developmental toxicity in general, and to malformations, specifically; and examination of developmental variations, embryolethality, and altered growth. None has demonstrated an unequivocal relationship between specific maternal and developmental toxicities: Developmental disruption appears not to result unconditionally from maternal toxicity. Maternal "stress" appears to have some impact on development but resists further definition at this time. Variations in association may be due to the extent to which maternal homeostasis has been compromised. Several quantitative approaches to relating maternal toxicity and developmental toxicity in animal systems (ie, relative teratogenic index, adult/developmental toxicity ratio) may provide the most satisfactory means of evaluating developmental toxicity testing for assessment of hazard.

The developmental toxicity of xylene and xylene isomers in the Hydra assay

Two laboratories tested multiple forms of xylene for their developmental toxicity hazard potential (A/D ratio) by means of the hydra assay. The three isomers, as well as a solution of mixed xylenes, all interfered with development (D) at or near to concentrations that also were toxic to adult (A) hydra. The A/D ratios ranged from 1 to 2 in hydra as they had in conventional tests made in pregnant laboratory animals. Each testing laboratory concluded that xylenes were not primary developmental hazards but coaffective agents capable of disrupting development only at or near to concentrations also toxic to adults. In each instance every xylene tested interfered with the same stage or developmental sequence and in a concentration-related manner. The hydra assay may be useful for establishing priorities to test agents in a more elaborate system, but substances less soluble than xylene may exceed the test's applicability.

Developmental phase-specific and dose-related teratogenic effects of ethylene glycol monomethyl ether in CD-1 mice

Several animal species have shown a teratogenic response to inhaled or ingested ethylene glycol monomethyl ether (EGME). The present study examined the developmental phase specificity and dose-response characteristics of EGME-induced embryotoxicity. Pregnant CD-1 mice (vaginal plug positive day = gestation Day [gd]0) received multiple or single doses of EGME by gavage between gd 7 and 14. Fetuses were examined on gd 18 for external and skeletal malformations. EGME was not maternally toxic after multiple doses of 250 mg/kg or a single administration of up to 500 mg/kg. EGME induced embryotoxicity as manifested by reduced gd 18 fetal weights and increased resorptions. The observed malformations were specifically related to the developmental stage at the time of exposure. Exencephaly resulted after EGME exposure between gd 7 to 10 whereas paw anomalies (syndactyly, oligodactyly, and stunted digit No. 1) predominated during later stages of development. Paw anomalies were maximal after administration on gd 11, and forepaws exhibited greater susceptibility than hindpaws. The no observed effect dose for the induction of digit malformations after a single administration of EGME on gd 11 was 100 mg/kg. At 175 mg EGME/kg digit anomalies were induced without any concurrent reduction in fetal body weight while at 250 mg/kg and above, digit anomalies occurred concurrently with reduced fetal body weight.

Regeneration by dissociated adult Hydra cells: a histologic study

Adult Hydra attenuata with vitally stained gastrodermal cells were dissociated into their component cells which were then randomly reaggregated into pellets by low-speed centrifugation. Representative examples of these preparations, which develop into normal adult hydra if left undisturbed, were examined fresh at low magnification and at higher magnification in fixed, stained, and sectioned specimens. The actual pellet stage lasts less than 1 hour because the adult ectodermal and gastrodermal cells rapidly sort themselves into an inner and outer layer and seem to secrete a new mesoglea immediately thereafter. The "embryo" becomes trilaminar and attains a central cavity by extruding a large amount of cellular debris at the end of the first day. At about this same time, new tentacles begin to differentiate from rapidly dividing and undifferentiated interstitial cells. Regulation of tentacle number and position occurs at the end of two days, and the body form is essentially reestablished within 60 hours by further differentiation of the hypostomes and body wall. Complete separation of the preparation into individual polyps does not occur until about 190 hours of development.

Relationship of developmental stage to effects of vinblastine on the artificial "embryo" of Hydra

Three milligrams of vinblastine sulfate per liter of culture medium prevented normal development of all reaggregated artificial hydra "embryos" at the 66-hour stage of regeneration. One milligram of vinblastine per liter of medium had no effect on development, and 100 mg did not markedly accelerate the developmental disruption beyond the pace of the 2 mg/liter concentration which was the lowest effective concentration. Treatment with vinblastine through the first few hours of development was sufficient to produce the toxic endpoint at 66 hours of incubation, but effects were delayed in appearance when treatment was initiated later in development. Early development of exposed preparations proceeded normally even in the presence of rather high concentrations of the test substance, and the pellet was resistant to the effects of vinblastine treatment begun after 24 hours of development. From these results, it would appear that interference with cell division is not the primary means by which vinblastine perturbs this developing system.